WIP: handle references in attrs and datasets

[magland]

This PR builds on #19

The goal here is to handle references in LindiH5Store and LindiClient such that the round-trip client handles refs in an equivalent way as the original h5py.

To illustrate, here are the tests that were added

def test_reference_attributes():
    print("Testing reference attributes")
    with tempfile.TemporaryDirectory() as tmpdir:
        filename = f"{tmpdir}/test.h5"
        with h5py.File(filename, "w") as f:
            X_ds = f.create_dataset("X", data=[1, 2, 3])
            Y_ds = f.create_dataset("Y", data=[4, 5, 6])
            X_ds.attrs["ref"] = Y_ds.ref
        h5f = h5py.File(filename, "r")
        with LindiH5Store.from_file(filename, url=filename) as store:
            rfs = store.to_reference_file_system()
            client = LindiClient.from_reference_file_system(rfs)

            X1 = h5f["X"]
            assert isinstance(X1, h5py.Dataset)
            X2 = client["X"]
            assert isinstance(X2, LindiDataset)

            ref1 = X1.attrs["ref"]
            assert isinstance(ref1, h5py.Reference)
            ref2 = X2.attrs["ref"]
            assert isinstance(ref2, LindiReference)

            target1 = h5f[ref1]
            assert isinstance(target1, h5py.Dataset)
            target2 = client[ref2]
            assert isinstance(target2, LindiDataset)

            assert _check_equal(target1[:], target2[:])

def test_reference_in_compound_dtype():
    print("Testing reference in dataset with compound dtype")
    with tempfile.TemporaryDirectory() as tmpdir:
        filename = f"{tmpdir}/test.h5"
        with h5py.File(filename, "w") as f:
            compound_dtype = np.dtype([("x", "i4"), ("y", h5py.special_dtype(ref=h5py.Reference))])
            Y_ds = f.create_dataset("Y", data=[1, 2, 3])
            f.create_dataset("X", data=[(1, Y_ds.ref), (2, Y_ds.ref)], dtype=compound_dtype)
        h5f = h5py.File(filename, "r")
        with LindiH5Store.from_file(filename, url=filename) as store:
            rfs = store.to_reference_file_system()
            client = LindiClient.from_reference_file_system(rfs)

            X1 = h5f["X"]
            assert isinstance(X1, h5py.Dataset)
            X2 = client["X"]
            assert isinstance(X2, LindiDataset)

            assert _check_equal(X1["x"][:], X2["x"][:])
            ref1 = X1["y"][0]
            assert isinstance(ref1, h5py.Reference)
            ref2 = X2["y"][0]
            assert isinstance(ref2, LindiReference)

            target1 = h5f[ref1]
            assert isinstance(target1, h5py.Dataset)
            target2 = client[ref2]
            assert isinstance(target2, LindiDataset)

            assert _check_equal(target1[:], target2[:])